5 - 3 How is the magma generated?

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Magma is a silicate melt produced by partial melting from mantle.

In the depths of the earth, the mantle is very hot, but it is applied high pressure. Thereby the mantle is in solid state. If this hot mantle comes up near the surface of the earth, then the pressure goes down and it would be partially melted. This is nothing but the magma.

Mantle High temperature, high pressure, solid
Magma High temperature but low pressure, liquid


Where the magma is generated
(= where a volcano is formed)

For the places where the mantle comes up,
Refer to the Figure on the previous page.
(1) Oceanic ridge Here, the plate tears and the hot mantle comes up from the depths and partially melts which erupts to form a submarine volcano and to become the oceanic crust.
Basaltic magmaD
(2) Island arc An example: the Japanese Archipelago
Mantle upwelling induced by subduction of oceanic plate at a trench iRefer to the Figure on the previous page.j
Andesite magma, dacite magma (which has high SiO2 contents and by slow solidification becomes granitic plutons which constitute the continental crust.)
(3) Hot spot Examples: the Hawaii Islands, Iceland
Mantle gushes up through a tube (pipe-like conduit) from a heat source located much deeper than the plate whose thickness is about 100km.


Mechanism of magma generation
The yellow line in the figure below shows the relation between the temperature and the depth under the ground. (Since the pressure depends on the depth, we can consider it as the relation between the temperature and the pressure.)
The temperature of the mantle is, in general, lower than its melting point. Hence the mantle is solid usually.
If there is a hot mass of mantle (temporally denoted by X in the figure), the specific gravity of the mass is slightly lighter than the surrounding part, and it gradually goes up (shown by the pink arrow in the figure).
Because the mantle is of low heat-conductivity (not easy to conduct heat), the mass of mantle hardly cools while it rises up (adiabatic rising). Continuing to rise up, it would finally begin to melt due to decompression. This is nothing but the magma generation. Continuing to melt, the volume of magma increases, and then the magma is separated from the mantle. Because the magma is light, it continues to go up. Finally it reaches the boundary between the mantle and the crust, and piles up there. This is just the magma chamber.
The magma made thus is basaltic magma of which composition is the same as that of the submarine crust or the lava flows on the Hawaii Islands.


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